The output characteristics of a semiconductor laser depend, in part, on its operating temperature. The wavelength of a laser's output signal may change gradually with temperature; however, at some temperature values, the wavelength of the laser's output signal may make discrete jumps. FIG. 1 is a graph illustrating an example of these discrete jumps as a function of device temperature. As illustrated in graph 100, these jumps occur when device temperature increases and the output signal changes from one longitudinal mode to another (referred to herein as “mode-hopping”).
Several solutions in the prior art exist to eliminate or reduce mode-hops. These solutions include passively compensating for temperature changes using different materials within the laser cavity (e.g., use of waveguide materials with negative thermal coefficients of refractive index (dn/dT), or cavity designs that utilize materials with temperature coefficients of expansion to get an athermal response). These passive, feedback-free methods of stabilization can compensate for uniform temperature changes, but cannot compensate for fluctuations within a laser cavity; furthermore, the passive tuning may have a limited temperature operating range due to lack of precision of tuning and lack of feedback. Furthermore, these passive solutions are difficult to employ in a photonic integrated circuit (PIC) due to the difficulty of using non-standard materials with -dn/dT and due to the difficulty in packaging a system with an external cavity with the required temperature coefficient of expansion.
Other solutions allow for changing the effective cavity length of the laser cavity when the device temperature changes. These solutions result in the mode continuously changing wavelengths without mode-hops as the temperature changes. These solutions have the disadvantage that as the temperature changes, the cavity has a large shift in the operating wavelength making it unsuitable for many implementations, such as wavelength division multiplexing (WDM).
Other prior art solutions heat the entire packaged PIC containing a laser to a fixed temperature, or heating the laser cavity only using on-chip heaters. Heating an entire laser or package is power-inefficient, and using this approach, the laser must be heated to the highest temperature in its designed operating range; furthermore, semiconductor lasers tend to operate more efficiently at lower temperatures, so heating a device to its maximum operating temperature compromises the power efficiency of the laser.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.